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The fabrication of devices based on III-V materials often requires a number of different reactive ion etching (RIE) processes that must be implemented sequentially. These processes are typically carried out in different RIE systems to avoid cross contamination. In this paper, we describe a multichamber RIE system configured to provide several sequential etch processes required for the fabrication of optoelectronic devices. This system has been used to fabricate InGaAsP/lnP ridge waveguide laser arrays with etched mechanical features that enable passive alignment of the lasers with single-mode fibers. Laser arrays with threshold currents as low as 20 mA have been processed with a high degree of uniformity. This system has also been used to develop a laser facet etch process based on CH4/H2/Ar chemistry. This process has been used to fabricate lasers with monolithically integrated rear facet monitors. These etched facet lasers have threshold currents comparable to lasers with both facets cleaved.

We report on the material synthesis and infrared optical properties of Nd doped lead bromide (PbBr2) and lead iodide (PbI2) bulk crystals. Commercial PbBr2 and PbI2 materials were purified through repeated solidification and horizontal zone refinement. After purification, Nd doped lead halides were synthesized and grown by Bridgman technique. Under optical excitation, Nd:PbBr2 samples exhibited several near-infrared emission bands centered at 816 nm, 891 nm, 963 nm, 1069 nm, 1183 nm, 1356 nm, and 1535 nm. The emission from Nd:PbI2 samples was similar to that of from Nd:PbBr2, but slightly shifted to longer wavelengths. The observation of 1540 nm emission from Nd3+ ions is unusual and reflects on the small non-radiative decay rates in the investigated halides. Lead halides have low maximum phonon energies, which reduces non-radiative decay due to multi-phonon relaxations. In contrast to Nd:YAG, Nd:PbBr2 and Nd:PbI2 exhibited efficient emission from the 4F5/2, 2H9/2 excited states, which are located only ∼1000cm−1 above the 4F3/2 level of Nd3+. Under 808 nm diode pumping, both samples also exhibited broad mid-infrared emission bands centered at ∼5.1 μm.

Bridged polysilsesquioxanes were synthesized by modifying the Si-O-Si polymeric network to produce highly nanoporous glasses for facile and uniform doping of nanoparticles. By taking advantages of void volumes created through the molecular modification technique, we designed and synthesized hexylene- and fluoroalkylenebridged polysilsesquioxanes doped with both Er+3 ions/CdSe nanoparticles for amplifier applications. Significant enhancement in fluorescence intensity at 1540 nm has been observed from the fluoroalkylene-bridged glass. Analysis by nuclear magnetic resonance (NMR) indicates a dramatically enhanced degree of condensation and a low level of hydroxyl environment in the fluoroalkylene-bridged hosts. The presence of CdSe nanoparticles, by virtue of their low phonon energy, also appears to significantly influence the nature of the surrounding photoluminescence environment of Er+3 ions in those organically modified hosts, resulting in the increased photoluminescence intensity.

The emission properties of Eu doped GaN thin films prepared by interrupted growth epitaxy (IGE) were investigated through excitation-wavelength dependent and time-resolved photoluminescence (PL) studies. Under above-gap excitation (333-363 nm) large differences were observed in the Eu3+ PL intensity and spectral features as a function of Ga shutter cycling time. The overall strongest red Eu3+ PL intensity was obtained from a sample grown with a Gashutter cycling time of 20 minutes. The main Eu3+ emission line originating from 5D0→ 7F2 transition was composed of two peaks located at 620 nm and 622 nm, which varied in relative intensity depending on the growth conditions. The room-temperature emission lifetimes of the samples were non-exponential and varied from ∼50 νs to ∼200 νs (1/e lifetimes). Under resonant excitation at 471 nm (7F0→5D2) all samples exhibited nearly identical PL spectra independent of Ga shutter cycling time. Moreover, the Eu3+ PL intensities and lifetimes varied significantly less compared to above-gap excitation. The excitation wavelengths dependent PL results indicate the existence of different Eu3+ centers in GaN: Eu, which can be controlled by the Ga shutter cycling time.

The magnetic properties of GaMnN, grown by metalorganic chemical vapor deposition, depend on the addition of dopants; where undoped materials are ferromagnetic, and n -type (Si-doped) and p -type (Mg-doped) films are either ferromagnetic or paramagnetic depending on dopant concentration. The ferromagnetism of this material system seems correlated to Fermi level position, and is observed only when the Fermi level is within or close to the Mn energy band. This allows ferromagnetism-mediating carriers to be present in the Mn energy band. The current results exclude precipitates or clusters as the origin of room temperature ferromagnetism in GaMnN.

We report on the growth and characterization of dilute magnetic semiconductor GaMnN showing ferromagnetism behavior above room temperature. GaMnN films were grown by MOCVD using (EtCp2)Mn as the precursor for in-situ Mn doping. Structural characterization of the GaMnN films was achieved by XRD, SIMS and TEM measurements. XRD and TEM confirmed that the films were single crystal solid solutions without the presence of secondary phases. SIMS analysis verified that Mn was incorporated homogeneously throughout the GaMnN layer which was ∼0.7μm thick. Ferromagnetic behavior for these films was observed along the c-direction (out of plane orientation) in a Mn concentration range of 0.025–2%. The saturation magnetization ranged from 0.18–1.05 emu/cc for different growth conditions. Curie temperatures of the GaMnN films were determined to be from 270 to above 400K depending on the Mn concentration. This dependence of Curie temperature on concentration of Mn in GaMnN indicates that the grown films are random solid solutions. SQUID measurements ruled out the possibility of spin-glass and superparamagnetism in these MOCVD grown GaMnN films. The grown films were electrically semi-insulating; however PL measurements showed that the films were still optically active after Mn doping. This study showed that the growth of III-Nitride films doped with Mn requires a small window of growth conditions that depend on growth temperature and (EtCp)2Mn flux to achieve ferromagnetism above room temperature, and the magnetic response of the film depends on the Fermi level position. We suggest that ferromagnetism occurs when the Fermi level lies within the Mn energy level which is 1.4 eV above the GaN valence band.

Dilute Magnetic Semiconductors (DMS's) posses a strong potential to make use of the spin of carriers in spintronic devices. Experimental results and theoretical calculations predict that GaN:Mn is a potential semiconductor material for spintronic device applications. The dependence of the room temperature ferromagnetic properties of GaN:Mn/GaN:Mg double heterostructures (DHS) on the Fermi level position in the crystal is demonstrated. Several GaN:Mn/GaN:Mg DHS are grown by metal organic chemical vapor deposition on sapphire. It is shown that initially paramagnetic films can be rendered ferromagnetic by facilitating carrier transfer through the GaN:Mn/GaN:Mg interface. Additionally, it is demonstrated that ferromagnetism depends on the thickness of the GaN:Mn and GaN:Mg layers. The carrier transfer process essentially changes the Fermi level position in the crystal. By choosing the right thicknesses for GaN:Mn and GaN:Mg an optimum DHS that exhibits room temperature ferromagnetism is grown. An identical structure, with the exception of insertion of an AlGaN barrier in order to obstruct the carrier transfer at the interface, results in paramagnetic films for AlGaN barriers thicker than 25nm. These results are explained based on the change in the occupancy of the 3d-Mn impurity band, and indicate that carrier mediation is the possible mechanism for the ferromagnetism observed in the MOCVD grown GaN:Mn material system. This is the first evidence that this material system responds to electronic perturbations, hence ferromagnetism observed is not due to secondary phases or spin glass behavior.

GaN and AlN thin films were implanted with gadolinium (Gd)atoms and characterized using deep ultra-violet (UV) photoluminescence(PL). The Gd-implanted samples were annealed at temperatures up to 1178K in a flowing N2 gas to facilitate recovery of implantation-related damage. Using the output at 195 nm from a quadrupled Ti:sapphire laser, narrow PL emission was observed at 318 nm from the Gd- implanted AlN thin films. This emission is characteristic of the lowest energy 4f transition of the trivalent Gd ion. A boarder emission band, also centered at 318 nm, was observed under excitation at 266 nm. No PL emission was observed from the Gd-implanted GaN thin films at either the bandedge or due to a 4f transition. The dependence of the UV emission on AlN sample temperature was systematically studied. The peak PL emission intensity decreased by less that a factor of 3 over the temperature range of 10 K to 300 K. Decay transients of the UV emission were measured indicating that the lifetime of this emission is very fast.

The GaN:RE phosphor development plays a major role in the GaN:RE AC thick dielectric electroluminescent (TDEL) device optimization. In this paper we report on EL devices fabricated using Eu-doped GaN red phosphors films grown by interrupted growth epitaxy (IGE). IGE consists of a sequence of ON/OFF cycles of the Ga and Eu beams, while the N2 plasma is kept constant during the entire growth time. IGE growth of GaN:Eu resulted in significant enhancement in the Eu emission intensity based primarily at 620.5nm. The increase in the material crystallinity observed with the IGE phosphors appears to be the dominant cause of the emission enhancement. Thick dielectric EL devices fabricated on glass substrates using IGE-grown GaN:Eu have resulted in luminance of ∼1000 cd/m2.

We report on the visible and infrared emission characteristics of Er-doped III-N lightemitting diodes (LEDs). Quantum well-like device structures were grown through a combination of metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) on cplane sapphire substrates. The dual stage growth process was used to take advantage of the high quality of AlGaN layers produced by MOCVD and in situ doping of Er during MBE growth. The multilayer structures were processed into devices and LEDs with different sizes and geometric shapes were produced. Electroluminescence (EL) was observed under either forward or reverse bias conditions. Visible and infrared spectra displayed narrow emission lines representative of the Er3+ system. The temperature dependence of the spectra, which were measured from 100K to 300K, showed a stability in the visible emission intensity but a sharp decrease in the infrared intensity at room temperature. Based on light output vs current measurements, estimates of the excitation cross-section were obtained for visible EL emission.

We report on the optical properties of Ho doped KPb2Cl5 (Ho: KPC) for potential applications as an infrared (IR) solid-state gain medium. The investigated crystal was synthesized from commercial starting materials of PbCl2, KCl, and HoCl3 followed by several purification steps including directional freezing, zone-refinement, and chlorination. The Ho: KPC crystal was subsequently grown by Bridgman technique. Following optical excitation at 885 nm, several IR emission bands were observed at room-temperature with average wavelengths at 1.07, 1.18, 1.35, 1.65, 2.00, 2.94, and 3.96 μm. The emission at 3.96 μm originated from the 5I5 -> 5I6 transitions of Ho3+ and was further evaluated for possible applications in mid-IR lasers. The decay time of the 5I5 excited state was measured to be 5.0 ms at room-temperature. The long 5I5 lifetime is favorable for laser applications and indicates that non-radiative multi-phonon relaxations are small in Ho: KPC. Based on a Judd-Ofelt analysis, the emission quantum efficiency was determined to be near unity resulting in a peak emission cross-section of 0.62×10-20 cm2 at 3.96 μm. A drawback for laser applications is the long decay time of the lower 5I6 state with a value of 4.8 ms . Since the 3.96 μm transition terminates in the 5I6 level, its long lifetime will lead to population bottlenecking, which limits possible mid-IR lasing to pulsed and quasi-cw operation.

Variable magnetic field Hall effect, photoluminescence (PL) and capacitance-voltage (CV) analysis have been used to study InN layers grown by plasma assisted molecular beam epitaxy. All three techniques reveal evidence of a buried p-type layer beneath a surface electron accumulation layer in heavily Mg-doped samples. The use of lattice-matched Yttria-stablized Zirconia substrates also provides evidence of a p-type layer.

We demonstrate optical transmission measurements performed on 1.2 μm thick GaMnN films grown by metalorganic chemical vapor deposition on (0001) sapphire substrates. According to the data acquired from these measurements, Mn forms a deep acceptor band at 1.4 eV above the valance band of GaMnN. Full width at half maximum of this absorption band increases from 107 meV to 198meV as the Mn concentration increases from 0.3% to 1.6 %; which indicates that this band becomes wider as the concentration of Mn increases in the lattice. A broad absorption band starting at 1.9eV and extending to the band edge of GaMnN was also determined. This was attributed to the transition from the Mn energy band to the conduction band edge of GaMnN. Absorption at both of these bands scales with the Mn concentration and thickness of the films. The effect of co-doping of GaMnN films with magnesium on the transmission spectra was also investigated. The absorption band initially observed at 1.4 eV was shifted to 1.6 eV as a result of introduction of Magnesium into the lattice of GaMnN. From these results we conclude that Mn is incorporated in the lattice and forms an energy band in the bandgap of GaMnN. The width of this energy band is also a function of the Mn concentration in GaMnN.

We report on metal organic chemical vapor deposition growth of GaMnN/p-GaN/n-GaN multilayer structures and manipulation of room temperature (RT) ferromagnetism (FM) in a GaMnN layer. The GaMnN layer was grown on top of a n-GaN substrate and found to be almost always paramagnetic. However, when grown on a p-type GaN layer, a strong saturation magnetization (Ms) was observed. Ms was almost doubled after annealing demonstrating that the FM observed in GaMnN film is carrier-mediated. To control the hole concentration of the p-GaN layer by depletion, GaMnN/p-GaN/n-GaN multilayer structures of different p-GaN thickness (Xp) were grown on sapphire substrates. We have demonstrated that the FM depends on the Xp and the applied bias to the GaN p-n junction. The FM of these multilayer is independent on the top GaMnN layer thickness (tGaMnN) for tGaMnN >200 nm and decreases for tGaMnN < 200 nm. Thus the room temperature FM of GaMnN i-p-n structure can also be controlled by changing Xp and tGaMnN in the GaMnN i-p-n structures.

In this study, we report on the diffusion of neodymium (Nd) and erbium (Er) into n-type and undoped GaN and subsequent measurements of the room-temperature (RT) magnetic and optical properties. The diffusion profile has been measured via secondary ion mass spectroscopy (SIMS) with rare-earth (RE) concentration yields of up to 1×1018/cm3. The ferromagnetic properties were measured using an alternating gradient magnetometer (AGM) giving a saturation magnetization (Ms) of up to 3.17emu/cm3 for the RE-diffused layer. The photoluminescence (PL) emission of the Nd-diffused and Er-diffused GaN is observable in the near-infrared (NIR) and infrared (IR) regions of the spectrum, respectively. The Nd-diffused GaN samples show NIR emission at 1064nm and 1350nm, while Er-diffused GaN samples have IR emission at 1546nm. This appears to be the first successful result of Nd diffusion doping into GaN crystals, and the first demonstration of above RT ferromagnetism involving GaN diffused with Nd. Details of our ferromagnetic and optical emission studies, related to the RE diffusion into GaN, are presented.

The elements of the lanthanide series, from Ce (atomic number 58) to Yb (atomic number 70), form a group of chemically similar elements that have in common a partially filled 4f shell. These so-called “rare earth” (RE) elements usually take on a 3+ ionic state (RE3+). Because the 4f electronic-energy levels of each lanthanide ion are shielded from external fields by 5s2 and 5p6 outer-shell electrons, RE3+ energy levels are predominantly independent of their surroundings.

The characteristic energy levels of 4f electrons of the trivalent RE elements have been investigated in detail by Gerhard Heinrich Dieke and co-workers and were reported approximately 30 years ago. The Dieke diagram showing RE3+ energy levels is a familiar tool of scientists and engineers working with RE elements. However, the history of RE elements goes back to the year 1787 in the small Swedish town of Ytterby near Stockholm and to the gifted amateur mineralogist and military man Lt. Carl Axel Arrhenius. Arrhenius discovered an unusual black mineral in Ytterby (perceived initially as much rarer in occurrence and in concentration than the common ores or earths of aluminum, calcium, etc.). Many new elements were discovered by various chemists upon analysis of this black stone and others like it. The names given to these elements are variations of the location where the first discovery was made: yttrium, ytterbium, terbium, and erbium. The history of RE elements is fascinating and involves many other famous names in science: Berzelius, Gadolin, Bunsen.

The properties of these elements and their multifaceted applications to science and industry are equally fascinating and have remained important to this day. Commercial applications of RE elements began after World War II, when their available quantity and purity were greatly enhanced by improved separation techniques developed as a part of the Manhattan Project. Until fairly recently, the main industrial application of RE elements has been in permanent magnets. The unpaired 4f electrons result in some RE elements having the highest magnetic moments of any element. The development and applications of RE magnets are reviewed in a very interesting article by Livingston3 in a previous MRS Bulletin issue. In this issue of MRS Bulletin, we have taken as our aim to review some of the properties and applications of RE elements relevant to photonics.

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